1. Field of the Invention
The present invention relates to a method for detecting a signal recorded on an optical recording medium, in which an edge detection method is used to reproduce digital data from an information pit pattern.
2. Description of the Related Art
In order to reproduce digital data from a pit pattern formed on an optical recording medium by mark-length recording, an analog signal is reproduced from the pit pattern 82 as shown in FIG. 9 and the analog signal is passed through a low-pass filter to be deprived of the DC component and a reproduced waveform 83 is generated. Then, inverted positions of data are discriminated from the reproduced waveform 83 and, thereby, the digital data is demodulated.
As a method to discriminate inverted positions of data from a reproduced waveform 83, there is known an edge detection method in which an established threshold voltage is superposed on the reproduced waveform to detect positions (edge positions) where the reproduced waveform 83 intersect with the threshold voltage, and the edge positions thus detected are taken as the inverted positions of data. In the edge detection method, the digital data is reproduced according to the detected edge positions, for example by taking an edge-to-edge distance as data 1, taking the next edge-to-edge distance as data 0, and so on.
As the threshold voltage for detecting the edge positions, the average value of the minimum voltage 85b and the maximum voltage 85a, [(85a+85b)/2], of the reproduced waveform 83 denoted by 85c in FIG. 9 is normally used. When such a threshold voltage is used, six edge positions E.sub.1, E.sub.2, E.sub.3, E.sub.4, E.sub.5, and E.sub.6 are detected as shown in FIG. 9.
When the average value of the minimum voltage and the maximum voltage of the reproduced waveform is established as the threshold voltage, difficulties arise. More specifically, in such cases where two pits are formed so close to each other that the adjoining reproduced waveforms overlap each other at their foot portions, where the rise or fall of the wave is deviated from its right position due to a variation in the recording power, or where, when an optical recording medium on which pits are formed by heat applied thereto (a magneto-optic recording medium or an optical recording medium of the WORM (Write Once Read Many) type using recording material which produces a change in phase or deformation when subjected to heat) is used, the pit form is distorted by the effect of accumulated heat and, hence, the reproduced waveform is deformed, such an irregularity is not compensated for and the distortion in the reproduced waveform directly affects the edge position, and as a result, for example the detected edge positions come to deviate from the inverted positions in the original digital data or the inverted positions are dropped in the reading, and, hence, the data corresponding to the original digital data becomes unobtainable. Accordingly, when the edge detection method, in which the threshold voltage is established as described above, is used, there is a limit in the improvement of the pit density and, especially when the method is used for reproducing data from an magneto-optic recording medium or an optical recording medium of a WORM type, it becomes necessary to severely control the laser intensity or laser irradiating time at the time of recording.